The invention relates to a method for calculating a physiological function parameter of a life form, whereby a measurement signal, which changes depending on the function parameter, is acquired with a measuring instrument, and whereby signal components of the measurement signal relating to another parameter that likewise influences the measured signal and having a characteristic frequency are separated out from the measurement signal by filtering. The invention is also directed to a corresponding arrangement, i.e., an apparatus, for calculating the physiological function parameter.
One such method is disclosed in U.S. Pat. No. 4,702,253, the teachings of which are incorporated by reference. The disclosed method is employed for identifying the respiratory activity of a patient.
In the disclosed method, blood impedance, which changes in view of patient respiration and in view of patient heart activity, is measured using electrodes arranged in the region of the heart of the patient and using a measuring instrument connected to the electrodes. A measurement signal generated by the measuring instrument is sampled by means of a sampling stage utilizing a sampling frequency of 100 Hz and the samples are supplied to a filter stage in which low-frequency signal components in the range from 0.05 Hz through 1 Hz are filtered out of the measurement signal. This filtering is supposed to remove signal components that relate to respiration. The respiration minute volume subsequently is identified from the filtered-out, low-frequency signal components and is utilized for frequency control of a heart pacemaker.
In the disclosed method, the extraction of the low-frequency signal components is undertaken with a band-pass filter having fixed limit frequencies. However, what is thereby left out of consideration is that the respiratory frequency can vary over a frequency range from approximately 0.09 Hz through 1.1 Hz, depending on the physical stress or emotional mood of the patient, and that the heart frequency can vary in a range from approximately 0.9 Hz through 2.5 Hz. The two frequency ranges thus overlap, so that the signal components utilized for the calculation of the respiration minute volume not only depend on the respiration but also depend on the heart activity, particularly given low heart beat frequencies. This can give rise to inaccurate calculations.